A hydraulic excavator composed of an upper revolving structure and a lower travelling structure has a multiplicity of hydraulic actuators such as hydraulic cylinders for moving a boom, an arm, a bucket and the like which constitute a working machine, and a travelling motor for rotating the right and the left tracks. In order to freely operate each of the actuators, two variable capacity type hydraulic pumps are usually mounted. As an alternative to the bucket fastened to serve as a working machine, an attachment such as a crasher or a hydraulic breaker or the like is sometimes fastened so as to perform an operation for crashing a structure or a rock. The aforesaid attachments respectively require different flow rates because the hydraulic devices for operating the attachments are different from one another. For example, a flow rate realized by two pumps is required in a case where a crasher is operated, while only a flow rate realized by one pump is required in a case where a hydraulic breaker is operated.
On the other hand, the above-mentioned attachments are usually used in such a manner that exchange of attachments for use in one hydraulic excavator is performed when the need arises. Therefore, the hydraulic excavator must be capable of easily switching the flow rate to be supplied to the attachment in order to be immediately adapted to the newly fastened attachment. Accordingly, in the conventional hydraulic excavator, a service valve 43 of two provided service valves 40 and 43 is, as shown in FIG. 6, switched so as to switch the flow rate between that realized by one main pump and that realized by two main pumps.
Referring to FIG. 6, five directional control valves for operating a revolving actuator, a boom-Hi actuator, a left-side service actuator, an arm-Lo actuator, and a left running actuator are connected in parallel to a variable capacity type hydraulic pump (hereinafter called a "main pump") 1. Five directional control valves for operating a right running actuator, a bucket actuator, a boom-Lo actuator, an arm-Hi actuator, and a right-side service actuator are connected in parallel to a main pump 5. Two pipes 41 and 42 connected to outlet ports of the right side service valve 40 are respectively connected to a hydraulic circuit extending from the left side service valve 43 to an actuator 19 so that they are joined together.
An attachment pedal 13 for operating the attachment is positioned in contact with a pilot-pressure control valve (hereinafter called a "PPC valve") 14, and a control pump 12 serves as a hydraulic pressure source for it. Two pilot circuits 44 and 45 extending from the PPC valve 14 are respectively connected to the left end portion and the right end portion of the service valve 43. Furthermore, branch circuits 46 and 47 are respectively provided for the two pilot circuits 44 and 45 and are respectively connected to the right end portion and the left end portion of the service valve 40 via pilot circuit directional control valves 48 and 49 having solenoids. The solenoids of the aforesaid pilot circuit direction control valves 48 and 49 are respectively connected to a selection switch 50.
In a case where, for example, a crasher is fastened as the attachment, a flow rate realized by two pumps is required in order to operate the crasher and therefore the selection switch 50 is switched on. As a result, the solenoids of the pilot circuit directional control valves 48 and 49 are excited, the branch circuits 46 and 47 of the pilot circuits 44 and 45 are thereby communicated, and the pilot pressure acts on the left end portion of the service valve 43 and the right end portion of the service valve 40 or acts on the right end portion of the service valve 43 and the left end portion of the service valve 40 by the operation of the attachment pedal 13. Thus, the total flow rate of the main pumps 1 and 5 acts on the actuator 19 which operates the attachment.
In a case where a hydraulic breaker is fastened as the attachment, only a flow rate realized by one main pump is required to operate the hydraulic breaker. Therefore, the selection switch 50 is switched off. As a result, the solenoids of the pilot circuit directional control valves 48 and 49 are demagnetized and thereby the branch circuits 46 and 47 of the pilot circuits 44 and 45 are closed. Thus, the pilot pressure acts on only the left end portion or the right end portion of the service valve 43 by the operation of the attachment pedal 13. As a result, the flow rate of the main pump 1 solely acts on the actuator which operates the attachment.
In order to control the discharge from the main pump according to the operation of each spool for the directional control valve, and more particularly, to control the discharge from the main pump to become minimum when each directional control valve is at the neutral position for the purpose of reducing wasteful flow, a relief valve 51 and an orifice 52 are provided for the main circuit in such a manner that they are connected to a flow-rate adjustment mechanism 53 of the main pump via circuits so that the discharge from the main pump is controlled.
However, the flow-rate switching circuit thus structured involves the following problems: (1) The two pilot circuit directional control valves 48 and 49 must be provided for the pilot circuit, and the two pipes 41 and 42 for realizing joining from the service valve 40 to the main circuit of the attachment actuator 19 must be provided. This leads to a fact that the hydraulic circuit becomes too complicated, causing the reliability of the hydraulic excavator to deteriorate. Furthermore, the number of the inspection processes undesirably increases and the manufacturing cost increases. (2) Since the quantity of oil to be supplied to the attachment actuator is switched in two stages, that is the quantity realized by the one pump and that realized by the two pumps, a fine adjustment of the flow rate cannot be performed.
Accordingly, an object of the present invention is to provide an apparatus for switching the flow rate for attachments having a simple hydraulic circuit, capable of switching a required flow rate for each attachment by a simple operation and also capable of finely adjusting the flow rate.